The present invention relates to silica particles having protuberances in their surfaces, which particles are to be incorporated in resin or rubber, and a process for producing the silica particles. The silica particles according to the present invention are useful as a filler for semiconductor sealant resin compositions, polyesters, engineering plastics and silicone rubber.
Various kinds of inorganic fillers, especially silica, are popularly used for the purpose of improving the properties of high polymeric materials. For instance, in IC sealants, a silica filler in a ratio of about 70 wt % are blended with the matrix resin such as epoxy resin, polyphenylene sulfide, polyimide, etc., for the purpose of adjusting the coefficient of thermal expansion. With a recent tendency toward higher integration of semiconductors and oversizing of chips, a request is rising for high-purifying of sealant, lower stress and improvement of crack resistance. However, with the present state of art, these requirements are not yet fulfilled sufficiently.
The conventional silica filler has been obtained by the following methods:
(1) melt pulverization method of natural quartz, PA1 (2) calcination and pulverization method of silica gel prepared by using sodium silicate, PA1 (3) decomposition method of silicon tetrachloride by oxyhydrogen flames, and PA1 (4) sol-gel method of alkoxysilane.
These methods, however, have the problems set forth below.
The method (1) has a problem in stable supply of natural quartz of good quality. It is also impossible with this method to reduce the content of the radioactive elements such as uranium, thorium, etc., which could cause erroneous operation (soft errors) of IC's. Silica produced by the method (2) is unusable as a silica sealant for highly integrated semiconductor devices because of probability of contamination with metal components such as sodium, aluminum, etc., or ionic impurities from sodium silicate used as a starting material or a neutralizing agent. The method (3) has problems of high production cost and difficulty in controlling particle size and shape. It is also impossible with this method to avoid inclusion of chlorine ions as an impurity. The method (4), although capable of producing high-purity silica by the purifying-treatment of the raw material, is complicated in production process, so that the produced silica becomes costly and its use is limited.
As regards the shape of silica filler, there has been mostly used square silica produced by the molten silica pulverization method. A composition containing the square silica, however, is poor in fluidity and its use tends to cause "hard errors" such as deformation of wiring or cracking of package due to stress of the sharp ends of the particles, so that recently spherical silica has come to be used usually for the improvement of fluidity and dispersion of stress. However such spherical silica still has the problems that it is relatively costly and that adhesion to resin is deteriorated, resulting in reduced strength of the molded package, since the silica particle surface is smoothened as a result of melting or sintering during the heat treatment in the production process.
As prior arts regarding silica particles having specific shapes, Japanese Patent Application Laid-Open (KOKAI) No. 182212/88 discloses the perfectly circular silica particles having a jagged surface, and U.S. Pat. No. No. 4,010,242 discloses a method of producing fine particles by using colloidal particles of 5 to 500 m.mu. as a starting material.
The silica particles proposed hitherto had the various problems in preparation and practical application, and accordingly the development of new silica particles free from the said technical problems such as concentration of stress, low fluidity and unsatisfactory adhesion to resin, which are observed in use of the conventional square or spherical silica as a filler has been required.
As a result of the present inventors, extensive and intensified studies for solving the said problems, it has been found that by (A) reacting hydrosilicofluoric acid, ammonium silicofluoride or a mixture thereof with ammonia in an aqueous medium to form a silica slurry, (B) separating a silica from the said slurry by means of a solid/liquid separation and (C) calcining, after optionally drying, the separated silica at a temperature of not less than 500.degree. C., the silica particles having a specific surface configuration can be obtained, which are excellently suited for use as a filler. The present invention was achieved on the basis of this finding.